1. Field of the Invention
The present invention relates to an image heating apparatus which heats an image borne on a recording material (such as a fixing apparatus which heats and fixes an unfixed image on the recording material as a permanent image, a heating apparatus which temporarily fixes an unfixed image on the recording material, a heating apparatus which modifies an image surface property such as gloss or the like by reheating the recording material bearing an image, or the like), and an image forming apparatus which is equipped with the above image heating apparatus.
2. Related Background Art
Conventionally, as a heating/fixing means to be provided in an electrophotographic image forming apparatus such as a copying machine, a printer, a fax machine or the like, it adopts a heat roller heating/fixing apparatus of a contact-heating type which has excellent heat efficiency and safety, a film heating heating/fixing apparatus of an energy-saving type which has a quick-start property (on-demand property) and suppresses power consumption as much as possible by suspending power supplying to the apparatus at standby time, and the like.
The heat roller heating/fixing apparatus includes a rotative fixing roller which acts as a heating member heated by a built-in heat source such as a halogen heater or the like and controlled to a predetermined temperature and a rotative pressure roller which acts as a pressure member being in contact with the fixing roller in a predetermined manner, and heats and fixes an unfixed image borne on the surface of a recording material by introducing the recording material into a pressure-contact nip portion (fixing nip portion) formed between the rotative fixing roller and the rotative pressure roller and then holding tight and conveying the recording material at the nip portion.
On one hand, the film heating heating/fixing apparatus has been proposed in Japanese Patent Application Laid-Open No. 63-313182, Japanese Patent Application Laid-Open No. 01-263679, Japanese Patent Application Laid-Open No. 02-157878, Japanese Patent Application Laid-Open Nos. 04-044075 to 04-044083, and the like. That is, in the film heating heating/fixing apparatus, a heat-resisting film which acts as a movement member is tightly pressed against a heating element by the pressure member and then driven, the recording material is introduced between the heat-resisting film and the pressure member at a pressure-contact nip portion (fixing nip portion) which is formed by the heating element and the pressure member through the heat-resisting film, and the recording material is then passed the fixing nip portion together with the heat-resisting film in the situation that the recording material is tightly in contact with the heat-resisting film. Thus, heat from the heating element is transferred to the recording material through the heat-resisting film, whereby the unfixed image is heated and fixed to the surface of the recording material.
In such heating/fixing apparatuses as above, in a case where recording materials (small size media) of which the width is narrower than the maximum sheet passing width of the apparatus are continuously passed and the heating/fixing operation to the passed recording materials is continued, a temperature of the area in the fixing nip portion where the recording material is not passed (called a non-sheet passing area) keeps increasing because there is no medium for absorbing heat in the non-sheet passing area. On one hand, a temperature of the area in the fixing nip portion where the recording material is passed (called a sheet passing area) is maintained to a predetermined fixing temperature by a temperature control system, whereby so-called a non-sheet passing portion excessive temperature increase phenomenon that a temperature difference between the non-sheet passing area and the sheet passing area in the fixing nip portion increases occurs (FIG. 12).
Here, it should be noted that the horizontal axis of the graph shown in FIG. 12 indicates a time, and the print on the small size sheets is continuously performed in the area described as “small size print.” Immediately after the small size print area, the print on the standard size sheets is continuously performed in the area described as “standard size print.” At this time, the temperatures on the non-sheet passing area and the sheet passing area are measured. That is, the graph shown in FIG. 12 indicates that the temperature increase of the non-sheet passing area is higher than that of the sheet passing area.
In the above non-sheet passing portion excessive temperature increase phenomenon, when the print is performed on the recording material (large size medium) of which the width is wider than that of the above recording material, a longitudinal direction temperature distribution in the fixing nip portion is indicated as in FIG. 13. That is, the temperature of the small size sheet passing area is maintained to a predetermined fixed temperature, an excellent fixed image can be obtained in the image area corresponding to the small size sheet passing area. However, due to the print of the recording material of the narrow width, the temperature of the non-sheet passing area has increased up to the temperature higher than the set temperature of the sheet passing area, whereby hot-offset due to excessive fixing occurs in the image area corresponding to the small size non-sheet passing area.
Moreover, an amount of the moisture evaporated from the sheet (paper) being the recording material increases when the temperature of the pressure roller extraordinarily increases in the non-sheet passing area, whereby there is a problem that the recording material slips because the conveyance force of the pressure roller is lost.
In a sequence for performing the print on the recording material of the wide width after performing the print on the recording material of the narrow width so as to prevent the hot off-set (Japanese Patent Application Laid-Open No. 11-143291), when a time elapsed from the end of the passing of the recording material of the narrow width to the beginning of the passing of the recording material of the wide width is short, the temperature in the nip at the time when the standard size (large size medium) print is performed is as indicated by a curve (1) of FIG. 14, whereby an excellent image in which the hot off-set can be prevented and sufficient fixability is secured can be obtained. However, when the time elapsed from the end of the passing of the recording material of the narrow width to the beginning of the passing of the recording material of the wide width is long, although the same temperature control as that in the case where the elapsed time is short in the small size sheet passing area is performed, the temperature of the pressure roller has become low, whereby there is a problem that the fixability of the sheet passing area resultingly deteriorates as indicated by a curve (2) of FIG. 14 because the fixing temperature is excessively decreased.